Previous proposals for frequency control include gas absorption cell techniques using a cell generally (but not always) outside the laser cavity. Operation of the cell at low pressures permits frequency stabilization based on the measurement of a variety of phenomena including Doppler broadened lines (for instance in CO.sub.2, OsO.sub.4, NH.sub.3, SF.sub.6, etc.), or super-narrow lines based on Lamb-dip methods. Detection can be effected by monitoring the reduction of transmitted power, using direct power detection, or by laser fluorescence methods which lock to the peak of the power curve by monitoring a portion of the frequency-modulated output power. Laser opto-acoustic methods have also been proposed and used.
Other methods previously proposed are sensitive to the laser frequency. An example is the optogalvanic method in which current changes are measured. This current change is a fuction of the plasma tube impedance, which in turn varies with laser frequency. A related method detects changes of plasma tube impedance by measuring the electric field outside and close to the laser tube. These methods are relatively simple to implement because the detection apparatus (but not the phenomenon) is outside the laser tube. However, the plasma discharge is maintained by a large dc current and fluctuations within the plasma generate electrical noise which is recorded by the detecting apparatus. If frequency modulation of the laser cavity length is used (as is usual) a large frequency swing is required to give a satisfactory level of signal to noise in the detected signals and this modulation in turn leads to spectral broadening of the laser output. Furthermore, harmonics of the mains electrical supply can be a significant contribution to the noise.
The extension of optogalvanic methods to radio-frequency-excited lasers has been made, but is poor in signal to noise ratio because of the high level of plasma discharge noise which then occurs.
The present invention seeks to address these problems and to provide an alternative arrangement for frequency stabilization.